NEWTOWN, Conn. (CBSNewYork) — As Newtown and the world pay tribute to victims of Friday’s massacre, police continued to work Monday to better understand the mind and motivation of the killer.

As CBS 2’s Dick Brennan reported, gunman Adam Lanza entered the school by blasting out the glass of one of its doors. People reported popping sounds coming from the front office.

The killer then headed down the hallway, toward the classrooms, where he later killed 20 children and six adults.

Now, investigators want to know if he tried to get out a message before the carnage. They are checking mailboxes and post offices.

“Do we have anything coming out of that house that might have been mailed before the shooting, the day of the shooting or the night before that could be a communique or videotape or some other message as they’ve seen in prior cases?” CBS News Senior Correspondent John Miller said.

“There are many people, including first responders, including town residents, including people right in this audience, that have broken hearts over this,” said State Police Lt. Paul Vance. “We’re going to do everything that it takes to ensure that we uncover every bit of evidence, that we examine every facet of it, that we conduct as many interviews with everyone that we need to do to paint a clear picture as to how and why this tragedy occurred.”

Investigators also want to know: what was going on in Lanza’s twisted mind. A former neighbor who babysat Lanza 10 years ago said his mother had a warning for him.

“His mom Nancy always instructed me to keep my eye on him at all times and never turn my back,” said former neighbor Ryan Craft.

The killer was also apparently awkward during his school years, and had a problem feeling physical sensations.

“You had to be very careful with him, keep an eye on him,” said Zoe Novia. “If you needed to get his attention, and you were to touch him and he wouldn’t have known really, you had to verbalize it if you were going to get his attention.”

“She was just very careful and cautious and responsible,” Ellen Adriani said.

“I’ve seen a lot of things in the media about her being a survivalist wacko,” said friend Russ Hanoman. “She did have guns, which she used very responsibly. She went shooting with a mutual friend of ours who is a retired New York City police officer.

Police were still blocking access Monday to the home where Lanza killed his mother.

Investigators in the meantime have reportedly been unable retrieve any data from two computers that Lanza smashed before going on his shooting spree.

A former security director at Newtown schools and his daughter remembered the gunman as very bright, but awkward.

“He had some social disorders and poor interpersonal skills that we felt strongly that we could develop. He was highly intelligent. But how do you inject him into the system and try to develop him in a sense socially?” Richard Novia said.

Meanwhile, investigators on Monday continued to gather evidence at Sandy Hook Elementary School, but they may get more help outside the school by talking to the two staff members who were shot but survived.

“That will shed a great deal of light on the facts and circumstances of this tragic investigation,” Vance said.